1. Field of the Invention:
The present invention relates to an improved process for the preparation of tri-(m-sulfophenyl)-phosphine.
2. Description of the Prior Art:
Tri-(m-sulfophenyl)-phosphine (hereinafter referred to as TPPTS) is a known compound useful as a water-soluble ligand permitting the formation of complexes with transition metals, for example rhodium, in water, and such solutions are well adapted as catalysts for the synthesis of aldehyde compounds by hydroformylation of olefins, as described in French Pat. No. 2,314,910.
A synthesis of TPPTS is also described in French Pat. No. 2,314,910. In Example 1 of this patent, the subject compound is prepared in the form of its sodium salt by sulfonation of triphenylphosphine (hereinafter referred to as TPP) utilizing oleum containing 20% by weight of sulfur trioxide (SO.sub.3). This gives mono-, di- and trisulfonated triphenylphosphine (hereinafter referred to as TPPMS, TPPDS and TPPTS, respectively) mixed with the oxides of these three compounds (the oxides being hereinafter referred to as OTPPMS, OTPPDS and OTPPTS, also respectively).
The sulfonation reaction is thereafter terminated by diluting the reaction mixture with water. This provides a hydrolysate which is rich in sulfuric acid and contains the sulfonated triphenylphosphine in the acidic form.
To isolate the latter compound in the form of a neutral salt, the hydrolysate is neutralized, for example with sodium hydroxide, thereafter the amount of sulfate anion present in the form of sodium sulfate in the aqueous medium is reduced by successive crystallizations, and the sulfonated triphenylphosphine is then crystallized in the form of its sodium salt by adding methanol and ethanol and subsequently evaporating the solvent. If the sulfonation of the triphenylphosphine is carried out at 30.degree. C. for about 20 hours it is possible to isolate, after various successive crystallizations, a mixture containing 80% by weight of the sodium salt of tri-(sulfophenyl)-phosphine and 20% by weight of the sodium salt of tri-(sulfophenyl)-phosphine oxide.
Further, it has been shown in Example 2 of Certificate of Addition No. 2,349,562 to said French Pat. No. 2,314,910 that in the TPPTS obtained in Example 2 of the basic patent, the sulfonate substitution is in the meta-position.
However, this mode of synthesis of TPPTS has disadvantages. First of all, the neutralization is accompanied by partial oxidation of the phosphine and the resulting oxide is undesirable in that the amount of inert diluent required during the downstream hydroformylation reaction is increased. Moreover, the amount of sodium hydroxide required for neutralization is substantial, since the hydrolysate is rich in H.sub.2 SO.sub.4. Consequently, the amount of impurities introduced with the sodium hydroxide is large. These impurities, which are subsequently retained in the final product, can interfere with the catalysis. Finally, a mixture of TPPTS, TPPDS and TPPMS is obtained from the sulfonation reaction and these compounds are very difficult to separate.
Published French patent application No. 82/14,862, filed Aug. 31, 1982, describes a process for the preparation of TPPTS which is similar to that described in said French Pat. No. 2,314,910, except that before neutralization with sodium hydroxide, the various sulfonated triphenylphosphines are extracted from the hydrolysate obtained after terminating the sulfonation reaction, by means of an extractant which is in particular selected from among the phosphonic acid esters, such as, for example, dibutyl butylphosphonate or the phosphoric acid esters, such as, for example, tributyl phosphate.
Thus, in Example 4 of said '862 patent application, after the TPP has been sulfonated with oleum containing 20% by weight of SO.sub.3 in H.sub.2 SO.sub.4, treatment with the extractant gives a mixture of 72.9% of TPPTS, 17.1% of TPPDS and 10% of sulfonated triphenylphosphine oxides. It can thus be seen that during the sulfonation stage TPPDS is formed.
It currently appears that among the various sulfonated triarylphosphines described in French Pat. No. 2,214,910, and its Certificate of Addition No. 2,349,562, TPPTS is the best of the ligands for forming a complex with rhodium, and that TPPTS is accordingly superior to TPPMS and to TPPDS.
However, if TPPTS is to provide the best results when used as a ligand, it is necessary that it should be of excellent purity and in particular that it should be free of TPPMS and TPPDS (in which the state of oxidation of the phosphorus is P.sup.III) and of OTPPMS, OTPPDS and OTPPTS (in which the state of oxidation of the phosphorus is P.sup.V).